1. Field of the Invention
The invention relates to a power supply system and, more particularly, to control over a power supply system that includes parallel-connected direct-current power supplies each including a battery and a step-up converter.
2. Description of Related Art
In recent years, the following power supply system is used as a power supply system for an electromotive vehicle. The power supply system includes two parallel-connected direct-current power supplies each including a battery and a step-up converter, and supplies the total output current of the two step-up converters to an inverter. Each step-up converter is to step up the voltage of the corresponding battery through PWM control for turning on or off switching elements, and the output current fluctuates. Therefore, when fluctuations in output currents from the two step-up converters overlap with each other, fluctuations in the output current of the power supply system increase, with the result that a capacitor that smoothes a direct current that is supplied to the inverter may become larger or vibrations and noise may increase. For this reason, there is suggested that fluctuations in output currents of the two step-up converters are cancelled by shifting the phases of carrier waves that are respectively used in PWM controls over the two step-up converters by 180° and, as a result, fluctuations in output current from the power supply system are reduced (see, for example, Japanese Patent Application Publication No. 2008-5625 (JP 2008-5625 A)).
Incidentally, when the temperatures of batteries are low, the charge-discharge characteristics decrease. Therefore, for example, when the electromotive vehicle is started up in a low temperature state, sufficient charge-discharge characteristics need to be ensured by quickly raising the temperatures of the batteries. However, there is an inconvenience that it takes time to raise the temperatures of the batteries by using such current fluctuations that occur in the step-up converters through PWM controls as described in JP 2008-5625 A.